A dental handpiece of a center-injection type has hitherto been used mainly for drilling and tapping the jawbone in dental implant surgery. A handpiece of this type is shown in FIG. 2 and disclosed in JP-8-322853-A (U.S. Pat. No. 5,816,803-A), which has bur sleeve 104 arranged vertically in the head casing 125 of the handpiece, and detachably holding therein cutting bur 100 having an axial through hole, such as a drill, for rotation. Water injection nozzle 120 connected to a cooling water feed source is inserted into the through hole from the upper end of the bur 100. The cooling water may be injected onto a cutting site through injection port 103 at the lower end of the bur 100 for drilling and tapping under water injection.
In such a conventional dental handpiece of a center injection type, when the injection port 103 of the bur 100 is pressed to the cutting site and blocked, the cooling water, such as physiological saline, flows up in a reverse direction through small gap 102 between the outer surface of the injection nozzle 120 and the inner surface of the bur 100, and leaks out of the bur 100 into the interior of the head casing. The saline from the injection nozzle 120 may cause rusting of the bearings 116, 116′ in the head casing and resulting in impairment of the durability of the handpiece.
In order to remedy such a drawback, packing 130 is provided for sealing a gap between the upper end of the bur 100 and the injection nozzle 120 extending out of the through hole 102 at the upper end, in order to prevent saline blowout through the gap.
However, even the gap between the injection nozzle 120 and the upper end of the bur 100 is sealed with the packing 130 against cooling water backflow, microvibrations generated in the treatment are transmitted through the gap 102 from the injection port 103 at the lower end to the upper end of the bur 100, which causes intrusion of contaminants, such as water, blood, or a drug solution, into the interior of the head housing 111. This results in rusting of upper and lower bearings 116, 116′ or gear 115 to significantly impair their durability.